It is known that high-voltage transformers are used for the transmission of electrical power between different voltage levels in power distribution networks. These transformers have for example for a nominal voltage level of 110 kV/380 kV an output of several 100 MVA. On account of the high voltages, a sufficiently large insulating distance must be maintained in each case between live components, which has the effect on the transformer side in particular that, when air is used as the insulating medium, there is an increase in the overall size. For this reason, transformers of such high voltage levels have been designed as oil transformers, which means that the actual transformer is arranged in an oil-filled tank, the oil serving both as an insulating medium and as a cooling medium. On account of the high insulation resistance of oil in comparison with air, the insulating distances, and consequently the overall size of the transformer, can therefore be advantageously reduced.
At the interfaces at which current-carrying conductors are led out from the insulating medium oil into another insulating medium, such as air, so-called bushings are used, in order thereby to provide the insulating distances required for different insulating media. However, bushings are also used whenever an electrical conductor is led from an enclosed space through a wall into a neighbouring enclosed space that is filled with the same insulating medium. Such an example is also encountered in the case of an oil transformer, that is wherever electrical conductors are led from the oil-filled internal area of an oil tank in which a transformer is arranged into an adjacent internal area of the same oil tank which is separated from the first area and from which for example cable connections are led out. Such bushings are known as oil-oil bushings. The separation of the different internal areas essentially serves for the formation of separate oil reservoirs. In the example mentioned, the side of the bushing that is facing toward the transformer is referred to as the input side and the side that is facing toward the cable space is referred to as the output side.
Oil-oil bushings are based on a standardized element known as a vacuum terminating element. This is a rotationally symmetrical, paraboloid-like, hollow bushing element of a solid insulating material, along the axial extent of which there is formed radially toward the inside a tubular bushing channel, the wall thickness of which is tapered conically toward at least one of its two ends. The form of the bushing element is essentially also comparable to that of a funnel, the funnel outlet being extended rearwardly into the widened region of the funnel. In the installed state, the widened side of the bushing element is facing away from the transformer and facing toward the cable space. The widened side of a bushing element is therefore also referred to as the output side and the side facing away from it is referred to as the input side.
Known oil-oil bushings based on such a standardized vacuum termination are laborious to install, since additional barriers are used on the output side in order to control the field strength occurring during operation in such a way as to be sure of avoiding an electrical breakdown through oil. Similarly, when there are relatively high voltages, an additional insulation is placed on the input side of the oil bushing in order to prevent partial discharges. According to known bushings, a wound paper insulation is used for this, involving a complex manufacturing process. Both the paper insulation and the barriers, which are made of pressboard, have a high risk of moisture absorption, which may lead to an impairment of their function in the oil transformer.